10 BULLETIN 275, U. S. DEPARTMENT OF AGRICULTURE. 



there will be fires from unpre veritable sources (lightning) , the risk in 

 extended cutting cycles increases from year to year with the growth 

 in value of the stand. The shorter the period in which a merchant- 

 able stand can be produced and turned into cash the smaller will be 

 the risk of loss. 



Loss from windfall is inevitable in many localities as a direct con- 

 sequence of selection cutting. It is clear that the cumulative risk 

 of a longer cycle is far in excess of a shorter one. Evidently, devas- 

 tating windstorms are more likely to occur during a period of 50 

 years than during a shorter one. Moreover, the loss in cash value, 

 even in single trees, from windfall is bound to be heavier in the older 

 stands. The larger, bulkier,, and therefore the more valuable the 

 tree, if sound, the more it is exposed to windfall. Falling trees in 

 brushing against their standing neighbors not infrequently cause 

 more or less serious wounds by bruising or tearing off the bark. 



The greatest risk, however, involved in the long cutting period is 

 from " deterioration," so called. 



In nature a steady process of elimination goes on. Of thousands 

 of seedlings springing up together in dense growths, comparatively 

 few reach sapling size, very few grow into poles, and fewer, even, to 

 standards. This natural thinning through competition in the fight 

 for soil, food, and light is furthered by various dangers to which the 

 young plants are exposed, such as from certain insects, foliage and 

 twig diseases, injury from mammals, snowbreak, frost, and drought. 

 Later, the surviving members of the stand are confronted with dangers 

 from the same and other sources, such as suppression, lightning, in- 

 sects, frost, and decay. The elimination of the weaker members 

 effects a selection of older, well-established individuals, some of which 

 may still suffer from the competition of their thriftier neighbors, but 

 are not forced out of the community, or, to use Fernow's 1 well- 

 chosen terminology, trees which are "oppressed," not suppressed. As 

 long as improvement cutting on a larger scale on the national forests 

 is impossible, the percentage of oppressed trees will depend upon the 

 length of the cutting cycle. Both these oppressed trees and their 

 more favored companions are exposed to dangers from which their 

 earlier life was free. 



Frost does a good deal of damage; here we are. less interested in 

 the damage done to the foliage or to the bark than in those more or 

 less long cracks in the wood which are caused by very low tempera- 

 tures. In cold weather the wood cylinder shrinks more in a tan- 

 gential than in a radial direction. Particularly at sudden low 

 temperatures, when the volume of the outer layers decreases rather 

 suddenly while the inner layers are still free from frost and have 



i Fernow, B. E. Forest terminology. In Forestry Quart., v. 3, no. 3, pp. 255-268, 1905. (See p. 266.) 



